There has been development of various types of radiation image capturing apparatuses including a so-called direct-type radiation image capturing apparatus in which a detection element generates an electric charge according to an irradiation dose of radiation such as X-ray and converts the electric charge into an electric signal, and a so-called indirect-type radioactive imaging device in which, after irradiated radiation is converted by a scintillator or the like into an electromagnetic wave such as visible light having a different wavelength, an photoelectric converting element such as a photodiode generates an electric charge in accordance with the energy of the electromagnetic wave which has been converted and irradiated and then converts the electric charge into an electric signal. It should be noted that a detection element in a direct-type radiation image capturing apparatus and a photoelectric converting element in an indirect-type radiation image capturing apparatus are collectively referred to as radiation detection elements in the present invention.
This type of radiation image capturing apparatus is known as a FPD (flat panel detector) and had been formed integrally with a supporting stand (or a bucky device) (for example, see Patent document 1), but, in recent years, a portable radiation image capturing apparatus, in which radiation detection elements and the like are stored in a housing, has been developed and put into practical use (see Patent documents 2 and 3, for example).
In the above radiation image capturing apparatus, for example as shown in FIGS. 3 and 7 described later, usually, radiation detection elements 7 are arranged on a detecting section P in a two dimensional manner (matrix form) and on each of the radiation detection elements, a switch formed of a thin film transistor 8 (hereinafter referred to as a TFT) is provided. The radiation image capturing apparatus is often configured so that: before radiation image capturing operation, that is, before irradiating the radiation image capturing apparatus with radiation outputted from a radiation generation device, a reset process is executed for discharging an excess electric charge remaining in each of the radiation image capturing apparatus 7, while appropriately controlling an ON/OFF state of the respective TFTs 8.
Then, after the reset process of each of the radiation detection elements 7 is finished, an OFF voltage is applied to the TFTs 8 by a gate driver 15b of scanning drive unit 15 via each of scanning lines 6 to turn all of the TFTs to the OFF state. When the radiation image capturing apparatus is irradiated with the radiation from the radiation generating device in this state, the electric charge of an amount according to a radiation dose are generated and accumulated in each of the radiation detection elements 7.
Thereafter, after an end of the radiation image capturing operation, as shown in FIG. 73, while sequentially switching between each of lines L1 to Lx of the scanning lines 5 to which an ON voltage for signal readout is applied by the gate driver 15b of the scanning drive unit 15, the electric charge accumulated in each of the radiation detection elements 7 is read out therefrom, and the read out electric charge is directed to charge-to-voltage conversion in a reading circuit 17 to be read out as image data.
However, in a case of the above configuration, an interface between the radiation image capturing apparatus and the radiation generation device that irradiate with radiation on the radiation image capturing apparatus needs to be appropriately constructed, so that on the radiation image capturing apparatus side, each of the radiation detection elements 7 is in a state in which the electric charge can be accumulated. The construction between the devices is not always simple. Also, when the device is irradiated with the radiation during the reset process of each of the radiation detection elements 7 on the radiation image capturing apparatus side, the electric charge generated by the irradiation flow out of each of the radiation detection elements 7, causing a problem of reducing conversion efficiency of the electric charge generated by the irradiation, that is, the conversion efficiency to the image data.
For this reason, techniques to detect radioactive irradiation by the radiation image capturing apparatus itself has recently been developed. And as a part of the techniques, the detection of the radioactive irradiation by the radiation image capturing apparatus itself has been developed using the techniques described in Patent documents 4 and 5 for example.
A radiation image capturing apparatus and a readout method of image data are disclosed in Patent documents 4 and 5, in which: while irradiating the radiation image capturing apparatus with the radiation, the lines L1 to Lx of the scanning lines 5, to which the ON voltage is applied from the gate driver 15b of the scanning drive unit 15, are sequentially switched to repeatedly execute the readout process for the image data outputted from the radiation emitting elements 7.
In this case, as shown in FIG. 74, when the ON voltage is sequentially applied to the respective lines L1 to Lx of the scanning lines 5 and a readout period is referred to as one frame, the electric charges generated in the radiation detection elements 7 by radioactive irradiation are read out separately in the readout process in each of the frames. Here, the readout period is a period of reading out the image data from each of the target radiation detection elements 7 for reading out the image data among all the radiation detection elements 7 arranged on the detecting section P.
For this reason, the image data of each of the radiation detection elements 7 is reconstructed as follows. Regarding the frames from the frame in which the radioactive irradiation is initiated to the frame in which the radioactive irradiation is finished, the image data read out in the respective frames before the next frame are added together for each of the radiation detection elements 7.
However, the inventors of the present invention knows that the following problem occurs in the cases of Patent documents 4 and 5, in which the readout process for the image data for each frame is configured to continue after the radioactive irradiation.
In this case, as shown in FIG. 75, the ON voltage is sequentially applied to each of the scanning lines 5 starting from the top and the readout process for image data for each frame is executed. And, for example, it is assumed that the irradiation is finished while the ON voltage is being applied to the scanning lines 5 in a portion ΔT which is a shaded area in FIG. 76. Here, in FIG. 76, the radioactive irradiation is performed on the entire region of the detecting section P and does not mean that the radioactive irradiation is performed only on the portion ΔT defined by the shaded area.
Thereafter, the readout process for the image data is carried on and then the image data for the respective frames is added together as described above, the image data being read out in two or three frames including the present frame. Then, when the image data for each of the radiation detection elements 7 is reconstructed, as shown in FIGS. 77A and 77B, shading unevenness appears in a radiation image p generated based on the reconstructed image data.
Specifically, for example, the radiation image p is generated based on each of the pieces of image data d which has been reconstructed when irradiating the entire region of the detecting section P of the radiation image capturing apparatus with the radiation of the same dose. And with respect to each of the pieces of image data d reconstructed along the extending direction of signal lines 6 (in the vertical arrow direction in FIG. 77A), the image data d of an image region δT becomes larger in value than the image data d in an image region A positioned thereabove and the image data d in an image region B positioned therebelow. Here, the image region δT corresponds to the position of the scanning lines 5 (i.e., shaded area ΔT in FIG. 76) to which the ON voltage is applied while being irradiated with the radiation.
This causes the image region δT to become somewhat darker than the image region A and the image region B. As described above, the problem is known that the shading unevenness in the radiation image p appears even if the radiation image capturing apparatus is irradiated with the radiation evenly.
The problem arises not only when the entire region of the detecting section P of the radiation image capturing apparatus is irradiated evenly with the radiation, but also when the radiation image capturing apparatus is irradiated with the radiation with an object interposed therebetween as in a practical case. Also in the latter case, the shading unevenness appears in the generated radiation image.
The reason is considered as follows, that the image data d in the image region δT becomes larger than the image data d of the image regions A and B.
That is, as shown in FIG. 78, when the image data di is read out from the radiation detection element 7i after the ON voltage is applied to the line Li in one of the scanning lines 5, simultaneously, an electric charge q leaks from other radiation detection elements 7, which are respectively connected to other lines L of the scanning lines 5, by small amounts through the respective TFTs 8. This causes the actual image data di read out as the image data of the radiation detection element 7i to become the image data corresponding to the sum value of an electric charge Q read out from the radiation detection element 7i and electric charges leaked from other radiation detection elements 7 through the respective TFTs 8.
Further, when the readout process is executed while irradiating the radiation image capturing apparatus 1 with the radiation, the radiation irradiating the radiation image capturing apparatus also irradiates the respective TFTs 8 or the irradiated radiation is converted into an electromagnetic wave by a scintillator to cause the electromagnetic wave to irradiate the respective TFTs 8, increasing the amount of the electric charge q leaked from the radiation detection elements 7 through the respective TFTs 8.
For this reason, in this case, the image data di read out as the image data of the radiation detection element 7i shown in the FIG. 78 becomes larger by the amount corresponding to the increased amount of the electric charge q leaked from the other radiation detection elements 7 connected to the same signal line 6. Thus, the image data d in the image region δT is considered to become larger than the image data d of the image regions A and B.
In the above case, the radiation image becomes difficult to be seen when the shading unevenness appears on the generated radiation image. And for example, in a case of utilizing the radiation image in a diagnosis and the like in a medical field, a lesion may be overlooked or misjudged if the shading unevenness and the lesion overlap with each other in the radiation image. Also, as in shown in FIG. 77B, the image data d of the image data δT, which has become larger than the image data d of the image regions A and B, is difficult to be corrected.
Therefore, applying the inventions described in Patent documents 4 and 5, a configuration is considered in which: the readout process for the image data is started before initiating the radioactive irradiation onto the radiation image capturing apparatus; and the readout process for the image data d is stopped at the time of initiating the radioactive irradiation, and the readout process for the image data d is not continued while irradiating the radiation image capturing apparatus with the radiation, as described in Patent documents 4 and 5.
In the above configuration, at the time of initiating the radioactive irradiation onto the radiation image capturing apparatus, the image data d read out from each of the radiation detection elements 7 connected to the scanning lines 5, to which the ON voltage is applied by the gate driver 15b of the scanning drive unit 15, becomes significantly larger than the image data d read out from each of the radiation detection elements 7 connected to the scanning lines 5, to which the ON voltage is applied, before the initiation of the radioactive irradiation.
Using the above phenomenon, as shown in Patent document 6 for example, the initiation of the radioactive irradiation can be detected by the radiation image capturing apparatus itself. In the imaging device in Patent document 6, the readout process for the image data is configured to start before initiating the radioactive irradiation onto the radiation image capturing apparatus, and to detect the radioactive irradiation at the time when the readout image data rapidly increases and exceeds a threshold value.
Further, in Patent document 7 for example, among all charge coupled devices (CCDs) as the radiation detection elements, the image data of the CCDs in multiple rows is read out simultaneously to improve the detection efficiency for detecting the radioactive irradiation.
Then, the configuration can be made in which: at the time of detecting that the read out image data rapidly increases and exceeds the threshold value, the application of the ON voltage to each of the scanning lines 5 by the gate driver 15b of the scanning drive unit 15 is stopped, and the readout process for the image data is not executed while irradiating the device with the radiation.